Implants for the treatment of osteoarthritis of the knee

ABSTRACT

A replacement knee implant has a femoral implant and a tibial implant, each of which are inset in a bone surface. The tibial implant is generally elongated with one end rounded and an opposite end conforming to the shape of the tibia, and is made of a metal alloy or a ceramic. The upper surface is dished while the lower surface is planar and can be parallel or sloped relative to the upper surface. The femoral implant for implementation in a femoral condyle is rounded such that, when implemented, the femoral implant is flush at the anterior and posterior sides and protruding away from the femur between the anterior and posterior ends. The femoral implant can have an elongated keel for extending into the femur, and can be made from a highly cross-linked polyethylene.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. Section 119(e) toProvisional Application Ser. No. 61/104,336, filed Oct. 10, 2008, andProvisional Application Ser. No. 61/154,980, filed Feb. 24, 2009, bothof which are incorporated herein by reference.

FIELD OF THE INVENTION

This disclosure relates to orthopedic knee implants, e.g., for thetreatment of osteoarthritis (OA).

BACKGROUND

The earliest implants for the treatment of OA of the knee consisted offixed metallic hinges at one extreme, and some type of interposition atthe other extreme. Interpositions included the use of fascia and other‘soft’ biological materials, and also metallic tibial plateaus andmetallic shells covering the distal femoral condyles. The ‘soft’materials could fail due to inadequate strength and the lack of fixationto the bone. The metallic components fared better. Tibial plateaus, suchas those in designs known as MacIntosh and McKeever designs, served tospace apart the bearing surfaces, thus potentially correcting thedeformity, and provided a smooth bearing surface for the femoralcondyles. Lack of fixation of the MacIntosh implant to the tibiasometimes allowed movement or even dislocation, in the McKeever design,the use of keels prevented this problem. The femoral resurfacing devicesfaced the dual problem of matching the surface geometry of the originalintact femur, and of shaping the distal femur to fit the implant. It isbelieved that obtaining a satisfactory range of motion, as well asstability, would be a problem in many cases due to the geometricalfactors noted above. Another issue with such devices, which were notrigidly fixed to the bone, was that there would be ‘interfacemicromotion’ leading to resorption of the adjacent bone and replacementwith fibrous tissue, leading to residual pain or aching.

This experience with interposition devices pointed to the benefits ofrigid fixation of the device to the bone, and to geometricalcompatibility within the joint. The question of whether pain resultedfrom the lack of fixation, or from the opposite side of the jointarticulating with a rigid metal surface, was not clear. Clues to thatquestion came from the hip, following the use of Austin-Moore implantsfor replacement of the femoral head. There was still some residual painfrom uncemented femoral components, but far less when the componentswere cemented. There does not seem to have been a series of knees wherea McKeever or a similar device has been fixed to the upper tibia usingcement or other means, hence the source of the pain remains in somedoubt in the knee. Another question with the use of an interpositiondevice in the knee is the potential wearing away of the cartilage (oreven bone) on the opposite side; because the rigidity of the metalcaused the contact stresses to be elevated. In the case of a medialmetallic tibial plateau, for a shallow bearing surface, the stresseswould be significantly elevated. Because in the intact knee, themeniscus would spread the load over a wide area. This fact suggests thatthe cartilage on the medial femoral condyle could wear out more quicklythan in a normal healthy joint.

An implant design of interest was the Gunston, designed in the late1960's by Frank Gunston from Winnipeg while working as a Fellow at JohnCharnley's Hip Center in Wrightington, England. A metal half-disc isembedded in the femoral condyle and just projects from it, and isarticulated on a plastic runner set into the medial plateau. There wasalmost complete conformity in the frontal plane, and partial conformityin the sagittal plane.

This configuration had several benefits. The sagittal curve of thefemoral condyle could be fairly closely reproduced given sufficientsizes, the slot in the femoral condyle gave a large surface area ofstrong cancellous bone for cemented fixation, and the tibial surfaceprovided a combination of AP and rotational stability and laxity.

The negatives were that a single sagittal femoral radius could notreproduce the reduced radius in high flexion and the increased radius inextension at the distal end of the femur, cutting a slot in the femoralcondyle sometimes endangered the strength of the bone on the outside,and the tibial plateau was of insufficient surface area such thatsinkage and loosening occurred, and uncovered bone often impinged onbone on the opposite condyle or abraded against the plastic.

The polycentric knee, as it became known, was used in thousands ofcases, especially at the Mayo Clinic, and provided good clinical resultsin a high percentage of cases.

In the early 1970's, Charnley produced an alternative implant, as shown,for example, in U.S. Pat. No. 3,953,899. Charnley used a thin flat metalplate with a single inner keel for fixation. This approach preserved ofmost of the strong cortical and sclerotic bone on the upper tibia tomaximize the fixation, especially important for a component which didnot necessarily cover the entire surface of the medial condyle. Charnleyalso designed a plastic runner that was embedded into the distal femur.The name ‘Load-Angle Inlay’ (LAI) described this particular feature ofCharnley's implant. The plastic runner was set so that it projectedabout 2 mm from the surrounding surface but was made to be flush at theanterior and posterior. This arrangement, where the plastic surface wasconvex and the metal surface was flat, was opposed to the convention ofmetal-plastic bearings, where the stationary and concave (or flat)component should be plastic and the moving surface metal. The rationalewas that the stresses in the convex plastic would be higher potentiallyleading to delamination wear, and the plastic might wear unevenly which,in the extreme, might cause a discontinuity in the knee motion.

In practice, wear testing would be needed to determine whether theparticular configuration used in the Charnley LAI would function wellenough for its application, although there appears to be no publicrecords in leading literature for such testing. Minns, Day, and Hardinge(1982) carried out a motion analysis of 29 patients, which indicatedsatisfactory function, with no mechanical problems being reported.

Another type of knee for medial OA was the unicompartmental or ‘uni’,introduced in the early 1970's. This design consisted of a metal femoralrunner onlaid over the entire arc of the femoral condyle from extensionto full flexion. The component design varied from having a curvedundersurface to contact the femoral bone after removing any residualcartilage, to a facetted surface requiring flat cuts to be made with anosteotome or saw. The fixation was usually augmented with one or moreposts, or blades, or a combination, using cement for immediate andlong-term fixation. The tibial component consisted of a hemicirculardisc of plastic, sometimes fitting inside a metallic baseplate. Thebaseplate helps prevent deformation of the plastic in the short andlong-term, and the fixation to the bone was more durable. Onedisadvantage is that more tibial bone needs to be removed to account forthe metal, paradoxically having an adverse effect on the fixation due tothe fact that the strength of the cancellous bone in the proximal tibiadiminishes with depth. As with the femoral component, fixation was bycement, and the undersurface had a combination of posts or blades. Onall designs, the upper tibial surface has been close to flat, providinglittle AP stability, in contrast to the medial surface of the intactknee. This round on flat, or at best cylinder on flat, configurationproduces high contact stresses. In long-term follow-ups, for net-shapemolded polyethylene, there has typically been a trough formed due towear and deformation, but no delamination.

An alternate design has been the meniscal bearing uni, where the femoralcomponent had a spherical bearing surface, with the back surface beingfaceted. The tibial component consisted of a flat metallic plate with apolished upper surface. A plastic meniscus was interposed between thetwo components and conformed with each. This produced low contactstresses, which minimizes deformation and wear. There was no constraintto AP displacement, other than friction.

The following refers to a study carried out in one of the inventors'laboratory, on the nature of the osteoarthritic lesions at the time oftotal knee replacement surgery. One of the main purposes of the studywas to determine if an early intervention procedure could have beencarried out involving only replacement of the medial side of the joint,rather than a total knee. The study of 100 cases was reported to theOrthopaedic Research Society's Annual Meeting in 2007. The predominantlesion of the medial femoral condyle was distal, which is the regionwhich undergoes weight-bearing in walking, by far the most frequentactivity of everyday living. The posterior condyle on the other hand wasfrequently preserved, which makes sense because it is onlyweight-bearing in the less frequent high flexion activities such asrising from a chair and steep stair climbing. The lateral condyle wasusually intact by visual appearance. A later study where the lateralhistology was examined, showed that the cartilage structure was normalfor that age group of individuals.

On the tibial side, the lesion on the medial plateau varied in location.On the other hand, the lateral side showed normal cartilage on that areacovered by the meniscus, but cartilage with some softening andfibrillation on the area not covered by the meniscus. Hence the medialside showed degeneration where a repair was necessary, while the lateralside was frequently normal such that it could sustain normalweight-bearing without need of replacement.

Most of the lesions occurred within the anterior half, the central half,or extended more than one half. A lesser number involved the posterior.When all of the lesions for all 100 cases were superimposed, it was seenthat all of the medial tibial plateau could be involved. This indicatedthat if a single style of tibial component was designed, it would needto cover the whole of the tibial plateau.

SUMMARY

This description relates to knee implants and methods for providing kneeimplants. The knee implant has two parts—a femoral implant and a tibialimplant, each of which has novel features and aspects.

A tibial implant is generally elongated with one end rounded and anopposite end conforming to the shape of the tibia at the anterior side.The upper surface is dished while the lower surface is planar and can beparallel or sloped relative to the upper surface. The lower surface canhave a protrusion, such as a keel. The tibial implant is typically about2-4 mm thick (at its thinnest point) and about 16-24 mm in width. Thetibial implant can be made of a metal or other material that allows theimplant to be made thin.

The femoral implant for implementation in a femoral condyle is roundedsuch that, when implemented, the femoral implant is flush at theanterior and posterior sides and protruding away from the femur betweenthe anterior and posterior ends. The femoral implant can have anelongated keel for extending into the femur, and can be made from ahighly cross-linked polyethylene.

The description also includes methods for providing the implants,including forming a channel in the tibia that can extent only partwayacross the top of the tibia, and introducing the tibial implant from theanterior side.

In some embodiments, the tibia is cut at and angle, such as about 5-10degrees relative to a long axis of the tibia, to allow a reduced bonecut.

Other features and advantages will become apparent from the followingdescription, drawings, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a general design concept.

FIGS. 2( a)-2(c) are side views of different cuts to the tibia.

FIG. 3 shows perspective views of embodiments of a femoral implant.

FIG. 4 shows views of embodiments of a tibial implant.

FIGS. 5 and 6 show steps for installing the implants.

DESCRIPTION

FIG. 1 shows a first general design concept including modifications to afemur 10 and a tibia 12. Femur 10 has a lower end with two condyles 14,16 for facing the tibia. A portion of condyle 16 has been removed andreplaced with an implant 18. The implants have a generally flat face onthe inside facing the femur and an outer side that is round to conformto the shape of the rounded condyle. The implant is preferably made of aplastic, such as a highly cross-linked polyethylene.

Tibia 12 has a generally D-shape portion that is removed. A tibialimplant 20 has a dished upper surface to provide anterior-posterior (AP)stability and to reduce contact stresses. The tibial insert preferablyhas at least one keel 22 on the underside for providing support,although two keels could be used. Tibial implant 20 can be formed in oneof several different ways, depending on whether there is a small orlarger varus (bow-legged) deformity.

If an isolated metallic tibial plateau is used, the bearing surfaceshould be shaped to give maximum conformity with the femoral condyles.In practice this means that there would be conforming surfaces in earlyflexion, but less conformity in flexion due to the diminishing sagittalradius of the medial femoral condyle. There is not the benefit, as inthe intact knee, of a meniscus that can change its shape according tothe shape of the femoral condyle itself. A conforming metallic componentwould require even more rigid fixation to the tibia than a shallowcomponent, because of the high shear and tilting forces that are likelyto occur. Another consideration for the shape of the upper surface of ametallic tibial plateau is the required stability on the one hand, andfreedom of motion on the other hand. In the intact knee, there are onlya few millimeters of AP laxity at all angles of flexion due to theactions of the cruciate ligaments. On the medial side, the stability isfurther augmented by the dishing of the tibial surface, the menisci, andthe medial collateral ligament. Hence from this point of view, themedial dishing of a metallic plateau would be an advantage regardingwear of the medial femoral condyle. Overall however, for durability andabsence of pain, replacement bearing surfaces would be needed for boththe femur and tibia, and each would need to be rigidly fixed to thebone. For kinematic compatibility, the contours of the artificialsurfaces should closely match those of the original femur.

For the treatment of the medial compartment in early OA, where thecruciate ligaments are intact or if adapted in cases where the anteriorcruciate is damaged, and where there is no significant varus deformity,there is an additional aspect. A nonlimiting example of a typical kneewhich would be suitable for this treatment is one where the arthriticlesions are localized on the distal medial femoral condyle, and on thecentral or anterior regions of the medial tibial plateau. The lateralside of the joint should be able to sustain normal weight-bearing whilethe patello-femoral joint should show only slight arthritic lesions atmost such that there is no significant pain deriving from thatcompartment. The patients benefiting would be those who still arepursuing an active lifestyle, with a typical age range from 50-65 years.The procedure is envisaged as performed through small incisions andinvolve much less trauma than a standard total knee replacement, andeven less trauma than a standard unicompartmental knee replacement.

Referring to FIGS. 2( a)-2(c), FIG. 2( b) shows a tibia with a bone cutwith a lower surface that is perpendicular to a tibial long axis, as isalso shown in FIG. 1. One alternative, as shown in FIG. 2( c) is to havethe lower surface tapered about 5°-10° in the frontal plane to match thebone deformity. This taper can considerably reduce the amount of strongtibial bone that is resected, especially at the inner side of the medialplateau. The surface is tapered to conform better to a tapered surfacein the damaged knee.

FIGS. 3-6 represent further embodiments that have some of the conceptsdiscussed above, and other features that are different alternatives fromthose above. FIGS. 3 and 4 are representative drawings of femoral andtibial implants, respectively, and FIGS. 5 and 6 are perspective viewsshowing processes for providing the implants to the knee.

FIG. 3 has figures that represent a portion of the femoral component.Femoral implant 30 a is designed to have a tight fit in a recess of thefemur, which has been machined through cartilage and into bone. Unlikethe implant shown in FIG. 1, it has a curve on the inner side that facesthe femur. The femoral implant can have various widths, such as 12 mm,16 mm, or 20 mm width as shown by implants 30 d, 30 e, and 30 f, and istypically 10-14 mm in width; and various thicknesses, such 6 or 8 mm, asshown in implants 30 b and 30 c. Different materials can be used for thefemoral implant, but preferably it is a plastic, such as ultra highmolecular weight polyethylene (UHMWPE).

In one embodiment, the femoral component is made from a wear-resistantpolymer such as highly cross-linked polyethylene, with a thickness of atleast 8 mm, an optional keel along the base 2-4 mm wide, where thecomponent is inset into the femoral condyle leaving 2-4 mm of bone oneach side. The component is sized to carry load from approximately 5degrees hyperextension to approximately 40-60 degrees flexion. It isflush with the cartilage at the anterior and posterior locations, andprojects 0.5-1 mm above the cartilage in the center, the projectiontapering down to zero at each end. The projection cause more of theweight-bearing to be in the component and less in surrounding cartilageat each side. The outer radii of the femoral component, in the frontalplane, is about 1-3 mm smaller than that of the tibial component formoderately close conformity and stability. The lower surface of thecomponent can be designed for osseointegration. All edges have a smallradius, such as 0.5 mm, to avoid stress concentrations of the boneinterfacing with the component.

An advantage of making the femoral component in a polymer is that thetibial component can be made from metal. A metal implant can be madethinner, thus requiring less tibial bone resection. However there arealternate material choices. A molded polyethylene can be used, or astiff polymer such as polyetheretherketone (PEEK). It is possible tomake the femoral component from metal, interfacing with polymer on thetibial side. These are the materials conventionally used today forunicompartmental replacements.

The implant can be fixed to the bone with polymethylmethacrylate cement(PMMA), which is commonly used in knee replacements. Another method isto bond a layer of a porous material such as porous tantalum to the baseof the plastic component and rely on subsequent bone ingrowth. The lowersurface could also be fused with a trabecular metal for ingrowthfixation. The side and lower surfaces can have grooves to help thebonding.

FIG. 4 illustrates different shapes and sizes of implants for use in atibia. The tibial implants can have one of several different forms andcan have several different shapes. A slot is cut into the tibia from theanterior side, and the implant is introduced anteriorly. The implant isassumed to be bonded to bone (after ingrowth) but not to cartilage. Asshown, the device can have different thicknesses, such as 6 mm or 8 mm,can use a single keel or a dual keel for support (1.5-2.5 mm wide and4-8 mm deep, with lower surface is designed for osseointegration), andcan have a width from about 12 to 24 mm, including widths of 10, 12, 14,16, 20, and 24 mm, or more typically, about 16-24 mm in width. Theimplant should have a thickness of 2-4 mm at its thinnest point,although higher thicknesses are available, such as 4-10 mm to cope withprevailing bone loss and deformity.

The tibial component can be made from a metal alloy, such as a Co—Cralloy or a surface hardened titanium alloy, or from a ceramic. Thetibial component is inset with 2-4 mm peripheral boundary of cartilage,and with meniscus preserved if applicable, where the bone preparationand component insertion is carried out from the anterior. Although notshown in FIG. 4, the upper surface can be dished with a radius of 50-90mm, or 60-90 mm, in the sagittal plane to limit the anterior-posterior(AP) displacements and provide AP stability, and a similar dishing inthe frontal plane to limit medial-lateral displacements and providestability, particularly at the interior to match the intercondylareminence of the anatomic knee. The radius can be about 60 mm anteriorlyand 90 mm posteriorly. The top surface can have a high polish for lowfriction and wear.

The tibial component can be made from different materials. If thefemoral component is made from metal, the tibial component can be madefrom a polymer, such as cross-linked polyethylene or moldedpolyethylene. It can also have a metal backing to provide greaterrigidity and reduce the deformation of the polymer.

Two keels are shown but a single keel is also a viable configuration,especially in cases with strong bone. Using one or two keels can beadvantageous because they can avoid the need for a deeper cut into thebone.

The fixation including PMMA or a porous surface, as well as the roundingof corners, as are used for the femoral component. Also, similar to thefemoral component, all edges of the tibial component have a small radiusto avoid stress concentrations.

FIGS. 5 and 6 show steps in an embodiment of a process. Referring toFIG. 5, an arthritic lesion on a femur 50 is typically in about thelocation shown at 52. The knee is exposed antero-medially (step a). Afemoral fixture 54 is placed over the lesion, and screwed into place.The femoral fixture is available in different sizes and shapes, e.g.,4-6 options (step b). A femoral burr 56 is used to work around fixture54. A keel slot is then made with a tibial burr. A depth collar of thefemoral cutter ensures a uniform depth of pocket (step c). The femoralfixture is removed (step d). The interior periphery of the pocket has a2-3mm radius, and the thickness can be about 4-6 mm. A femoral component57 can have multiple shapes, e.g., 6-8. In this embodiment, it has onekeel. The dimensional variables are the sagittal radii, the AP lengthand the ML width. The component is preferably made from plastic such asUHMWPE. Fixation can be with acrylic cement, or with a fused-in porousmaterial. The femoral component is fixed. The anterior and posterior areflush as shown at 58, while the implant 60 can be protruding in betweenthe ends as shown at 62 (step e).

Referring to FIG. 6, a tibial fixture 64 is pinned to an anterior of atibia 66 and is aligned with femoral component 57 (step f). The two tagsof the fixture 64 determine a depth of cut. A tibial burr is used tomake a pocket and a keel slot (step g). The tibial fixture is removed,so the interior periphery of the pocket has a 1-2 mm radius (step h).

The inserted tibial component 70 is compatible with preserving themeniscus, which is released anteriorly to allow access to the componentThe tibia component can come in different sizes and shapes, e.g., 4-6.The dimensional variables are the sagittal radii, the AP length and theML width. The component is preferably made from a metal, such as Co—Cralloy. Fixation can be with acrylic cement, or with a fused-in porousmaterial. The thickness of tibial component is about 2-3 mm. The tibialcomponent is fixed, ensuring that the boundaries are flush or slightlyrecessed relative to the surrounding cartilage surfaces (step i). Asshown here, the tibial implant is generally elongated with one endrounded and an opposite end designed to conform to the shape of thetibia.

In this embodiment, the tibial implant can incorporate aspects of thefeature of FIG. 2 where the lower surface is tapered 5-10°.

The compressive stresses on the bone at the base of the recess werecalculated using finite element analysis for the normal anatomic knee,and for the different versions of the femoral and tibial components. Thecriterion was that the normal stresses were the baseline against whichto compare the stresses after implantation. If the stresses were higher,that would imply that there was a possibility of compressive bonefailure, which would impair the fixation and durability of the implant.On a comparative basis, implants with lower stresses are preferred, allelse being equal.

The stresses for the anatomic knee were less than for all of theimplants analyzed, including on the femur and tibia. The stresses wereapproximately inversely proportional to width.

For the femur, the stresses were similar whether plastic or metal wasused, for both 6 mm and 8 mm thick components. For the tibia, there wassome advantage to using metal for the thinner component. For the tibia,there was a major reduction of stresses using a keel. Using two keelsproduced a further significant decrease. Rounding the edges of thecomponents, including the keels, avoided stress concentrations at thoselocations. By insetting components, versus seating on a straight-acrossresection, was in transmitting shear stresses were transmitted down theperipheral bone contact, hence reducing the stresses on the lowersurface of the bone.

1. An implant for the knee comprising: a tibial implant that isgenerally elongated with one end rounded and an opposite end conformingto the shape of the tibia at an anterior side, the implant designed forbeing surrounded by bone on three sides when inlaid in the bone surface,the upper surface being dished in the sagittal plane and in the frontalplane, and the lower surface having means for assisting in fixation tothe bone.
 2. The implant of claim 1, wherein the tibial implant is about16-24 mm in width.
 3. The implant of claim 1, wherein the tibial implantis made from one of a cobalt-chromium alloy and a titanium alloy.
 4. Theimplant of claim 1, wherein the dishing of the tibial implant has asagittal radius between about 60-90 mm.
 5. The implant of claim 1,wherein the means for assisting includes one of an anterior to posteriorkeel and grooves.
 6. The implant of claim 1, further comprising, incombination, a femoral implant for implantation in a femoral condyle,the femoral implant being rounded such that, when implanted, the femoralimplant is flush at the anterior and posterior sides and protruding awayfrom the femur between anterior and posterior sides up to about 0.5 -1mm, the femoral component being at least 8 mm in thickness and made froma plastic.
 7. The implant of claim 6, wherein the femoral implant has anelongated keel for extending into the femur.
 8. The implant of claim 6,wherein the femoral implant includes highly cross-linked polyethylene.9. The implant of claim 6, wherein the femoral implant is about 12-14 mmin width such that when implanted there is bone on each side.
 10. Animplant for the knee comprising: a femoral implant substantiallyconforming in vivo to a distal femoral condyle, the femoral implantbeing rounded such that, when implemented, the femoral implant is flushat the anterior and posterior sides and protruding away from the femurbetween anterior and posterior sides up to about 0.5 mm-1 mm.
 11. Theimplant of claim 10, wherein the femoral implant has an elongated keelfor extending into the femur.
 12. The implant of claim 10, wherein thefemoral implant includes highly cross-linked polyethylene.
 13. Theimplant of claim 10, wherein the femoral implant is about 16-24 mm inwidth and about 12-20 mm in length.
 14. The implant of claim 1, thetibial implant being made from one of a metal alloy and a ceramic. 15.The implant of claim 1, the tibial implant being about 2-4 mm thick atits thinnest point.
 16. The implant of claim 10, the femoral implantbeing at least 8 mm in thickness.
 17. The implant of claim 10, thefemoral implant including at least one plastic surface.